Iron is produced in furnaces into which iron ore, a fuel, usually coal or coal char, and oxygen are introduced. The ore, primarily Fe.sub.2 O.sub.3, undergoes a preliminary reduction to FeO by contact with carbon monoxide and carbon in the top part of the furnace or in a chamber above the furnace, and the FeO is further reduced to metallic iron by contact with carbon monoxide and other reducing gases in the lower part of the furnace. The carbon monoxide is produced by partial combustion of the fuel, which is introduced into the lower part of the furnace together with oxygen. During the course of the process, a layer of molten iron forms at the bottom of the furnace and a layer of slag, produced from slagging agents introduced into the furnace with the fuel and from impurities contained in the ore, forms on top of the molten iron.
The hot waste gas exiting the furnace generally contains, by volume, about 10 to 35% carbon monoxide, about 2 to 15% hydrogen, about 40 to 55% carbon dioxide and the balance mostly nitrogen, water vapor, sulfur compounds (SO.sub.x, H.sub.2 S, etc.) and nitrogen oxides NO.sub.x). Ore and ash solids are also entrained in the waste gas. The waste gas is generally passed through particle separator to remove the solids, then passed through a waste gas boiler to recover heat energy, then treated to remove sulfur and nitrogen oxides, then combusted to recover the fuel value of the carbon monoxide contained in the gas, and finally discharged into the atmosphere.
In order to improve the efficiency of the ironmaking process, a stream of inert gas, such as nitrogen, can be bubbled through the molten iron to agitate the melt and thereby improve contact between any unconverted FeO in the melt and the carbon monoxide and carbon. Introduction of nitrogen into the furnace has certain disadvantages. Since nitrogen is inert and has no fuel value; additional fuel must be introduced into the furnace to provide the energy necessary to heat the nitrogen to the desired operating temperature. Additionally, the nitrogen increases the volume of waste gas issuing from the top of the furnace, thus increasing the size requirements of the waste gas treating equipment used with the furnace. Furthermore, the high temperature inside the furnace causes some of the nitrogen to be converted to nitrogen oxides, thereby increasing the concentration of pollutants in the waste gas, and the presence of nitrogen in the waste gas lowers the combustible value of the gas.
Methods of improving the operating efficiency of ironmaking processes are continuously sought. The present invention provides a significant improvement in the operating efficiency of these processes by significantly reducing, or entirely eliminating, intentional introduction of nitrogen into the furnace, by enhancing the efficiency of fuel conversion in the furnace efficiency, and by reducing the amount of sulfur in the molten iron by reducing fuel consumption.